The present application is directed to lighting devices, and more particularly to ballast circuitry for discharge lamps.
Electronic ballasts utilize electronic circuitry to stabilize current for fluorescent lamps, high-intensity discharge lamps, and the like. Electronic ballasts may be started using one of several starting techniques, including “instant” start, “rapid” start, and “programmed” start. The instant start technique starts a lamp in the short term, because it starts and operates the ballast without preheating a cathode associated therewith, which results in low cost in ballast design but wears out the lamp more rapidly than other starting protocols due to the violent nature of the starting method. The rapid starting technique starts the ballast and heats the cathode concurrently, resulting in a relatively long start time while mitigating the deleterious effects of a cold start on the lamp's cathode. Finally, the programmed start technique employs a cathode preheating period at low glow discharge current which increases the lamp's life for frequency switching applications.
Description of a ballast which includes an instant program start configuration for use with parallel lamps has been set forth in U.S. Pat. No. 7,193,368, titled, Parallel Lamps With Instant Program Start Electronic Ballast, to Chen et al., issued Mar. 20, 2007. This ballast takes advantage of the beneficial aspects of a program start ballast (e.g., longer lamp life) and combines it with the advantages of an instant start ballast (e.g., quick start time) to produce an improved lamp ballast wherein parallel lamps are driven.
U.S. application Ser. No. 11/645,939, titled, Switching Control For Inverter Startup And Shutdown, filed Dec. 27, 2006, which includes a low cost shutdown circuit.
Both U.S. Pat. No. 7,193,368 to Chen et al., and U.S. application Ser. No. 11/645,939 to Chen et al. are both hereby incorporated by reference in their entireties.
Drawbacks such as described above, which have startup and shutdown circuitry, are that the shutdown control circuitry acts on only one of the bipolar junction transistor (BJT) switches. Therefore, when the complete turnoff of that particular BJT switch does not occur in a short time interval, an imbalance will exist on series connected electrolytic capacitors of the ballast. As a result of the imbalance, one of the two series electrolytic capacitors of the ballast is subject to overvoltage, leading to the failure of the overloaded capacitor.